I. Field of the Invention
Present invention relates to methods and apparatus for syntonizing a passive atomic resonator with respect to a reference frequency.
II. Description of the Prior Art
The most common types of atomic clocks, also referred to as atomic frequency standards, include an oscillator whose frequency is accurately controlled through the utilization of a passive atomic resonator. The term "passive" as used herein is employed to distinguish such resonators from "active" resonators, such as atomic hydrogen masers, which are actually atomic oscillators. Typically through some combination of synthesizer and multiplier, the oscillator provides an interrogation signal which is used as a microwave frequency input to the passive atomic resonator. The passive atomic resonator has an inherent or natural frequency, and upon introduction of the interrogation signal, the passive atomic resonator provides an output error signal which is a function of the difference between the interrogation signal frequency and the natural frequency of the passive atomic resonator. This output error signal is utilized to control the frequency of the oscillator and thereby to control the frequency of the interrogation signal until the interrogation signal is locked to the natural frequency of the passive atomic resonator. The oscillator thereafter provides a highly accurate and stable frequency output signal for use as a frequency standard or atomic clock.
One well-known form of prior art passive atomic resonator employs a vapor of the alkali metal, rubidium, which has a natural hyperfine transition frequency of about 6835 megahertz. Such prior art passive atomic resonators employ a photo detector to provide the output error signal for control of the frequency of the oscillator. By processing the error signal from the photo detector, a control signal can be developed for adjusting the frequency of the oscillator in such a manner as to lock the interrogation signal frequency to the hyperfine frequency of the resonator.
Other forms of prior art passive atomic resonators which have potential application to the present invention include non-rubidium alkali vapor devices, beam tube devices, saturated absorption devices, and trapped ion devices, all of which provide an output error signal indicative of the difference between the frequency of an input interrogation signal and the frequency of the resonator.
The natural frequency of passive atomic resonators may fluctuate as a consequence of a number of influences. For example, in rubidium passive atomic resonators, fluctuation in the frequency of such resonators may occur due to variation in the intensity of a pumping light utilized to excite the alkali vapor atoms. Discoloration of glass and variations in temperature, electronic circuit parameters, and gas pressure are each capable of introducing shifts in the frequency of the resonator.
Provisions are made to allow for adjustment of the frequency of passive atomic resonators to thereby negate the adverse effect of shifts in the frequency thereof. Again, as an example, in rubidium passive atomic resonators a magnetic bias field, or C-field, is applied to separate the Zeeman levels of each hyperfine state of the vapor atoms. This separation, which is a function of the magnetic bias field, allows for selective altering of the frequency of the resonator, as is well-known to those skilled in the art.
Calibration, or frequency syntonization, of an atomic clock is accomplished in the prior art by removing the clock from operation and carefully measuring its output frequency with respect to an established reference frequency source. Prior art apparatus for measuring the frequency of an atomic clock is typically bulky and complicated. Once the absolute frequency of an atomic clock is measured, the variance of this frequency from the desired frequency is used to calculate the amount of variation to the magnetic bias field of the clock needed to establish a new magnetic bias field which will syntonize the frequency of the resonator to the reference frequency. The success of this frequency alignment, or syntonization, is determined by remeasuring the absolute frequency of the atomic clock after the magnetic bias field correction.
Typically, such prior art frequency syntonization methods may only be accomplished as part of a laboratory recalibration process. Moreover, such prior art frequency syntonization methods may have to be repeated several times in order to achieve a satisfactory frequency setting, and accordingly, such methods require on the order of 10 to 15 minutes for completion.
It is, therefore, an object of the present invention to provide methods and apparatus for rapidly and efficiently setting the frequency of a passive atomic resonator into close frequency agreement, syntonization, with a reference frequency established by a master external reference signal.
Another object of the present invention is to provide methods and apparatus for quickly performing syntonization of a passive atomic resonator in the field, particularly to provide frequency syntonization of a moderate precision tactical atomic clock.
A further object of the present invention to provide frequency syntonization hardware simple enough to be included within the simplest atomic clock.
Still a further object of the present invention is to provide apparatus and methods for controlling syntonization of the passive atomic resonator of an atomic clock automatically.
Still another object of the present invention is to provide methods and apparatus for frequency syntonization of a passive atomic resonator without the need for external frequency measurement.
Additional objects and advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out in the appended claims.